In many industrial applications, a liquid is conveyed through a piping system that can be extended over an elaborate three-dimensional network having bends, tees elbows, etc., held in place by braces or hangers. The possibility exists that a pressure pulse can be generated in the liquid, where the instantaneous liquid pressure might rapidly increase by a factor of 1.5-10 or even more at times. This pressure pulse is transmitted in the form of a shock wave through the liquid at sonic velocity (for example, 4,000-8,000 ft. per sec.) through the piping system.
The travelling pressure shock wave can damage equipment along the piping system and/or can damage the braces supporting the piping system. The effect this shock wave has on the piping system can also be amplified because of a resonant condition, occasioned, for example, when opposite ends of the same relatively straight section of piping are simultaneously exposed to a positive increase and a negative decrease in the wave pressure. Moreover, pressure intensities can be amplified where parallel flow paths reunite to combine the many waves in each path.
This particular phenomenon has critical consequences in liquid metal fast breeder reactor designs where molten sodium is used as a first coolant conveyed through a piping system; and where water is used as a second coolant that at a heat exchanger interfaces with the sodium. In the event a large leak should occur at this interface allowing contact between the sodium and water, a rather violent almost explosive-like reaction can occur which would generate a large pressure shock wave.
Because sodium is highly corrosive and is maintained at temperatures in the range of 400.degree.-600.degree. C., many conventional shock absorbing or energy-dissipating devices prove ineffective or are not usable. For example, a rupturable disc is frequently used in a T-connection off the sodium piping system to separate the system from a secondary or reaction products handling system. A reverse-buckling thin spherical shell is located in the T-connection with its convex side subjected to the fluid system, and a cutting-knife setup is placed immediately near the concave side of the disc. Upon the occurrence of a sufficiently intense pressure pulse in the sodium system, the disc is reverse flexed and ruptured, and the sodium can escape through the ruptured disc into the secondary system. This reduces the overall pressure of the sodium in the piping system, and also reduces the transmitted pulse, both in magnitude and duration. Even so, the transmitted pulse can cause significant damage to the piping system unless the system is strengthened and reinforced to take the increased loads.
One major drawback to any diversion of sodium from the piping system (by rupture disc actuation) is the reduced capacity for cooling the reactor. Moreover, the rupturable disc system cannot distinguish between a shock wave generated by a sodium-water reaction and one generated by a severe seismic event. Consequently, the disc must be sized to withstand seismic events of probable intensity, which thereby limits the sensitivity of the system. Another drawback is that the ruptured disc must be replaced and the sodium that has been diverted into the secondary system must be pumped back into the piping system.
Another commonly used shock absorber or pressure suppressing device is a surge tank connected by a tee off the main liquid line. The surge tank can be formed with a piston movable in a cylinder to expand and accept the diverted liquid, or the tank can have a pressurized gas overspace that is compressed in accepting the diverted liquid. Because of the reflection of the energy collected in the surge tank, the capacity to dissipate shock energy is limited as the energy basically is commonly returned back to the system after some delay. This system, however, can attenuate the intensity of the pressure, and moreover has appeal over the rupturable disc system in that it need not be replaced once it has been activated. Also, the accumulated liquid can be pumped or drained by gravity back into the main piping system after the pressure surge has been dissipated.
Another type of shock or energy absorber commonly used in some liquid piping systems is an expandable rubber membrane formed off a tee in the piping system, which retains the liquid at one volume when the liquid is under stabilized pressure conditions but which increases in volume upon a surge of pressure to dissipate or absorb some of the shock wave energy. However, the energy absorbed by the rubber membrane is also stored in the membrane so that once the pressure wave has passed, the energy is released back into the piping system. Of even greater importance, the rubber membrane cannot function at the temperature and pressure limits (400.degree.-600.degree. C. and 100-200 psi) of molten sodium, and thus would be impractical in the reactor cooling conditions.
This same deformable absorber concept is also employed in some applications as a sealed hollow collapsible rubber tube sized smaller than and located within the piping system. The sealed tube is thereby collapsed upon a pressure surge. However, this device, being internal, has limited capacity and impedes normal liquid flow through the piping system. Furthermore, as previously noted, rubber cannot be used in the high temperature and corrosive environments of molten sodium.